pH represents the effective concentration (activity)
of hydrogen ions (H+) in water. This concentration could be expressed
in the same kind of units as other dissolved species, but H+ concentrations
are much smaller than other species in most waters. The activity of hydrogen ions
can be expressed most conveniently in logarithmic units. pH is defined as the
negative logarithm of the activity of H+ ions:

pH = -log [H+]

where [H+] is the concentration of H+ ions in moles per
liter (a mole is a unit of measurement, equal to 6.022 x 1023 atoms). Because
H+ ions associate with water molecules to form hydronium (H3O+)
ions, pH is often expressed in terms of the concentration of hydronium ions.
In pure water at 22° C (72°
F), H3O+ and hydroxyl (OH-) ions exist in equal
quantities; the concentration of each is 1.0 x 10-7 moles per liter
(mol/L). Therefore, pH of pure water = -log (1.0 x 10-7) = -(-7.00)
= 7.00. Because pH is defined as –log [H+], pH decreases as [H+]
increases (which will happen if acid is added to the water). Since pH is a log
scale based on 10, the pH changes by 1 for every power of 10 change in [H+].
A solution of pH 3 has an H+ concentration 10 times that of a solution
of pH 4. The pH scale ranges from 0 to 14. However, pH values less than 0 and
greater than 14 have been observed in very rare concentrated solutions.

The pH of water can be measured with a pH meter, which is an electronic device
with a probe. The probe contains an acidic aqueous solution enclosed by a glass
membrane that allows migration of H+ ions. The electrical potential
of the glass electrode depends on the difference in [H+] between
the reference solution and the solution into which the electrode is dipped.
pH can also be measured with pH paper or by adding a reagent (indicator solution)
to the water sample and recording the color change.

Carbon dioxide (CO2) enters a water body from a variety of sources,
including the atmosphere, runoff from land, release from bacteria in the water,
and respiration by aquatic organisms. This dissolved CO2 forms a
weak acid. Natural, unpolluted rainwater can be as acidic as pH 5.6, because
it absorbs CO2 as it falls through the air. Because plants take in
CO2 during the day and release it during the night, pH levels in
water can change from daytime to night. For an example of how pH typically varies over a daily cycle, select here.

Geology and Soils of the watershed

Acidic and alkaline compounds can be released into water from different types
of rock and soil. When calcite (CaCO3) is present, carbonates (HCO3,
CO3-2) can be released, increasing the alkalinity of the
water, which raises the pH. When sulfide minerals, such as pyrite, or "fool’s
gold," (FeS2) are present, water and oxygen interact with the
minerals to form sulfuric acid (H2SO4). This can significantly
drop the pH of the water. Drainage water from forests and marshes is often slightly
acidic, due to the presence of organic acids produced by decaying vegetation.

Drainage from Mine Sites

Mining for gold, silver, and other metals often involves the removal of sulfide
minerals buried in the ground. When water flows over or through sulfidic waste
rock or tailings exposed at a mine site, this water can become acidic from the
formation of sulfuric acid. In the absence of buffering material, such as calcareous
rocks, streams that receive drainage from mine sites can have low pH levels.

Air Pollution

Air pollution from car exhaust and power plant emissions increases the concentrations
of nitrogen oxides (NO2, NO3) and sulfur dioxide (SO2)
in the air. These pollutants can travel far from their place of origin, and
react in the atmosphere to form nitric acid (HNO3) and sulfuric acid
(H2SO4). These acids can affect the pH of streams by combining
with moisture in the air and falling to the earth as acid rain or snow.

The U.S. Environmental Protection Agency (U.S. EPA) sets a secondary standard
for pH levels in drinking water: the water should be between pH 6.5 and 8.5
Secondary standards are unenforceable, but recommended, guidelines.

CDPHE-WQCD regulations state that waters used for primary recreation (including such activities as swimming, rafting, and kayaking) should have pH values between 6.5 and 9.0.

CDPHE-WQCD regulations state that waters classified as
"Class 1 Cold Water Aquatic Life" or "Class 1 Warm Water Aquatic
Life" should have pH values between 6.5 and 9.0.

Very high ( greater than 9.5) or very low (less than 4.5) pH values are unsuitable
for most aquatic organisms. Young fish and immature stages of aquatic insects
are extremely sensitive to pH levels below 5 and may die at these low pH values.
High pH levels (9-14) can harm fish by denaturing cellular membranes.

Changes in pH can also affect aquatic life indirectly by altering other aspects
of water chemistry. Low pH levels accelerate the release of metals from rocks
or sediments in the stream. These metals can affect a fish’s metabolism and
the fish’s ability to take water in through the gills, and can kill fish fry.

Other Information about pH

The term "pH" was originally derived from the French term "pouvoir
hydrogène," in English, this means "hydrogen power." The term pH
is always written with a lower case p and an upper case H.

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